Photoionization study of simple organic compounds using the R-matrix method

ABSTRACT R-matrix calculations using the Quantemol-N expert system on the photoionization of acetylene, ethylene, methanol, and dimethyl ether for photon energies ranging from near-threshold to 40 eV were performed and reported here. Partial low-energy photoionization cross-sections have been calculated for ionization out of the two outermost valence orbitals ($1\pi _u$ and $3\sigma _g$) leading to the $\mathrm{X}^2 \mathit{\Pi _u}$ and $\mathrm{A}^2 \mathit{\Sigma _g^{+}}$ states of the $\mathrm{C}_2 \mathrm{H}_2^{+}$ ion, four valence orbitals ($1b_{3u}$, $1b_{3g}$, $3a_{g}$, and $1b_{2u}$) corresponding to the $\mathrm{X}^{2} \mathrm{B}_{3u}$, $\mathrm{A}^{2} \mathrm{B}_{3g}$, $\mathrm{B}^{2} \mathrm{A}_{g}$, and $\mathrm{C}^{2} \mathrm{B}_{2u}$ states of the $\mathrm{C}_2 \mathrm{H}_4^{+}$ ion, four ionic states $\left(2 a^{\prime \prime -1}\right) { }^2 \mathrm{A}^{\prime \prime }$, $\left(7 a^{\prime -1}\right) { }^2 \mathrm{A}^{\prime }$, $\left(6 a^{\prime -1}\right) { }^2 \mathrm{A}^{\prime }$, and $\left(1 a^{\prime \prime -1}\right) { }^2 \mathrm{A}^{\prime \prime }$ of $\mathrm{CH}_3 \mathrm{OH}$, and at last exclusively for the ionization from the first valence orbital ($2b_1$) of dimethyl ether. We have presented high-resolution cross-section data near the photoionization threshold for the first time. These cross-sections are computed by incorporating electron correlation effects and large resonance structures, which have not been included in previous studies. We also did the calculations using different active spaces and target states to see if the current results were consistent and how they changed when the model was changed. Our best results, compared with available experimental and theoretical data, find reasonable agreement that confirms the reliability of our calculations. It is worth noting that previous studies have not fully accounted for autoionization, multichannel coupling, and correlation–polarization, which may be the reason for the slight variations in the results.